Degradation of signal-to-noise ratio (“SNR”) as well as Bit energy to noise ratio (“Eb/No”) or Bit energy to interference ratio (“Eb/I”) or Carrier to-Interference (“C/I”) ratio may occur due to interference signals that may typically appear in a communication channel with a desired received signal carried along a transmission medium (e.g. coax, unshielded conductor, wave guide, open air or even optical fiber or RF over fiber) at the receiver input. These degradations and interferences may occur in communications technologies including, for: example, CDMA, EVDO, and WCDMA, respectively operating under the 3G CDMA 2000 and UMTS standards, as well as TDMA and GSM. Signal quality attenuation and its resulting Eb/I degradation may limit bandwidth over a transmission medium. Interference from outside signals within the frequency range of a communication channel may also reduce Eb/I of the channel and reduce the amount of data the channel may carry and/or may block or saturate the communication channel receiver. In some situations, it may cause a loss of a full frequency channel. Additionally, in some situations, Eb/I degradation, e.g., due to interference signals, may render a communication (traffic or control) channel inoperative and may even degrade the base station capacity.
In order to improve the Eb/I of signals being transmitted over long distances, and accordingly to augment the transmission distance and/or data rate, signal repeaters may be placed at intervals along the transmitting path. Repeaters are well known and may be used for optical, microwave and radio frequency (RF) communication systems. Repeaters have been used as part of cellular transmission systems to extend the range of coverage between a cellular base station and a cellular handset.
However, the use of a broadband repeater (e.g., pass wide range of operating frequencies) for one or more channels at one or more frequencies within a frequency range of the spectrum (“Operating Band”) (e.g. 800 MHz, 900 MHz, PCS, Public Safety, or any other network operating band) may produce noise interference to the network. Furthermore, interference signals present in the vicinity of the repeater, and within the frequency range of one of the communication channels to be repeated, may also be repeated and amplified by the repeater, effectively reducing the Eb/I of a communication channel to be repeated. Interference signals may further introduce interferences to a base station receiver that may cause, for example, a CDMA cell shrink, or may lower the base station capacity. Turning now to FIG. 1A, there is shown a spectral diagram exemplifying frequency bands or channels (e.g. 1.25 MHz each) of a first cellular operator which may be used within the frequency range of the “Operating Band” in a CDMA channel. Also shown in FIG. 1A is a narrow band interference signal of, for example, 25 KHz bandwidth, introduced by some outside source, within the frequency range of a second communication channel of the first cellular operator. The interference signals may reduce the Eb/I of one or more communication channels, and the use of a conventional repeater may serve to boost the interference signal and reduce the Eb/I of the communication channel with which it is interfering. Repeaters, as part of a Cellular network system are typically installed geographically far from base station coverage. The interference may appear in the vicinity of the repeater receiver horizon or in the vicinity of the base station receiver horizon, and may create a problem from the horizon to the base station receiver. Repeaters and/or base stations are not generally capable of solving these problems.
FIG. 1B shows a spectral diagram exemplifying possible channel frequency bands that a second cellular operator may use in the same geographic location as the first cellular operator of FIG. 1A. However, the second operator of FIG. 1B may use communication channels with a narrower bandwidth (e.g. 200 KHz) than those used by the first operator in FIG. 1A, and may also use two or more communication channels having adjacent frequency bands, to create the required traffic capability. For example, one channel may be primarily used for traffic and the other channels may be primarily used for access. All channels may generally need to be operational, so that the communication may be adequately performed. This scenario may typically occur in broader areas, e.g., between states, countries, or in areas close to water such as lakes and seas. Base stations that are installed near a water source may receive an undesired signal from far base stations or even from far countries because water is an excellent conducer of RF waves. Accordingly, as shown in FIG. 1A, the frequency bands of the operator B, e.g., the third channel in FIG. 1B, may overlap and interfere with the frequency bands of operator A, e.g., with the second channel in FIG. 1A. Additionally or alternatively, as shown in FIG. 1B, the frequency bands of the operator A, e.g., the fourth channel in FIG. 1A, may overlap and interfere with the frequency bands of operator B, e.g., with the sixth channel in FIG. 1B.
Another exemplary scenario of communication without using a repeater may occur in the context of an outdoor environment. In the outdoor environment there may be interferences in the operating base station receiver, e.g., interference signals such as TV stations or other cellular operators. These interferences may affect the base station, for example resulting in cell shrinkage and/or lower base station capacity. When a communication channel having an interference signal, e.g., as shown in FIG. 1A or in FIG. 1B, is received by a cellular base-station, the interference signal may have an adverse effect on the base station receiver. Either the receiver may not be able to extract data from the channel, or in some cases, the receiver may fully block the receiver capacity or control channel.
An interference signal may be of a fixed nature, having relatively fixed frequencies and amplitudes. Alternatively, an interference signal may be intermittent and of an unstable nature.
As should be clear from FIGS. 1A and 1B, the center frequencies and frequency-band-size of communication channels used by one or more wireless service operators/providers in a given geographic location, may vary substantially, and thus each channel may have considerably different filtering requirements when passing through a repeater and/or at the input stage of a base-station.